In the field of optical devices, such as, displays, light emitting diodes and solar cells, various proposals have been recently made for the purposes of energy conservation and of improvement in light utilization efficiency. For example, there is a known method for increasing the aperture ratio of a liquid crystal display. In that method, a transparent planarization film is formed to cover a TFT element and then pixel electrodes are formed on the planarization film (see, Patent document 1). Also, in order that the aperture ratio of an organic EL device may be increased like that of a liquid crystal display, it is proposed to change the light extraction method from “bottom emission type” into “top emission type” (see, Patent document 2). Specifically, a bottom emission type of organic EL device comprises a substrate, a transparent pixel electrode provided thereon and a luminescent layer formed on the electrode by vapor deposition, and the emitted light is extracted from the substrate side. In contrast, a top emission type of organic EL device comprises a TFT element, a planarization film provided thereon for covering the element, a transparent pixel electrode formed thereon and a luminescent layer placed thereon, and the light emitted from the luminescent layer is extracted from the side opposite to the TFT element.
According as display devices have been getting enlarged, improved in resolution and in image quality and developed in 3D imaging, signal delay in wiring has been becoming a serious problem. When rewriting speed (flame frequency) of image information is accelerated, signals inputted to the TFT element are shortened. However, since images are required to be displayed in high resolution, there is a limitation on such enlargement of the wiring width as is intended to reduce the wiring resistance. In view of that, it is proposed to increase the wiring thickness enough to solve the problem of signal delay (see, Non-patent document 1).
As materials for the planarization film formed on the TFT substrate in the above proposal, it is known to adopt an acrylic resin and a quinone diazide compound in combination (see, Patent documents 3 and 4). Those materials are not drastically impaired in their properties at as high a temperature as 200° C. or above, but they gradually come to decompose at 230° C. or above and consequently the film thickness may be decreased. Further, when the substrate is treated at a high temperature, the film tends to be colored to reduce the transparency thereof. Because of that, the above materials cannot be adopted to form a transparent film employed in a process for providing a coating layer thereon at a high temperature by means of an apparatus such as PE-CVD. Also for producing an organic EL device, they cannot be said to be optimal materials because decomposed products thereof have negative influences on the luminance efficiency and lifetime of the organic EL device. In addition, acrylic resins modified to be heat-resistant generally have high permittivity, and hence an insulating film thereof has such a large parasitic capacitance as to increase the power consumption, to cause delay of liquid crystal driving signals and, as a result, to lower the quality of displayed images. Even if the insulating film is made of materials having high permittivity, the capacitance can be reduced by, for example, thickening the film. However, that coping method is not preferred because it is generally difficult to form a thick and uniform film and further because it is necessary to use a large amount of the materials.
Meanwhile, polysiloxanes, particularly, silsesquioxanes are known as materials of high heat resistance and of high transparency. A silsesquioxane is a polymer comprising a trifunctional siloxane structural unit RSi(O1.5), which can be considered to have an intermediate chemical structure between an inorganic silica structure (SiO2) and an organic silicone (R2SiO). This polymer is such a specific compound as is soluble in an organic solvent but forms a cured product characteristically having high heat resistance almost comparable to that of inorganic silica. Further, there is also a known material comprising a polysiloxane and a quinone diazide compound in combination (see, Patent document 5). According to the document, this material has high transparency and can form a highly transparent cured film on a substrate. The transparency of the cured film is reported not to lower even if the substrate is treated at a high temperature. Patent document 6 discloses another material comprising a siloxane polymer and an acrylic resin.